Gas-assisted laser ablation

ABSTRACT

An improved method for laser processing that prevents material redeposition during laser ablation but allows material to be removed at a high rate. In a preferred embodiment, laser ablation is performed in a chamber filled with high pressure precursor (etchant) gas so that sample particles ejected during laser ablation will react with the precursor gas in the gas atmosphere of the sample chamber. When the ejected particles collide with precursor gas particles, the precursor is dissociated, forming a reactive component that binds the ablated material. In turn, the reaction between the reactive dissociation by-product and the ablated material forms a new, volatile compound that can be pumped away in a gaseous state rather than redepositing onto the sample.

This application is a continuation application from U.S. applicationSer. No. 12/828,243, filed Jun. 30, 2010, and claims priority from U.S.Provisional Application 61/232,780, filed Aug. 10, 2009, which arehereby incorporated by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to fabricating microstructuresby laser micromachining, and more particularly, to laser micromachiningusing an etchant gas.

BACKGROUND OF THE INVENTION

Removing material from a substrate to form microscopic or nanoscopicstructures is referred to as micromachining, milling, or etching. Lasersbeams and charged particle beams are two particular technologies usedfor micromachining. Each has advantages and limitations in variousapplications.

Laser systems use several different mechanisms for micromachining. Insome processes, the laser is used to supply heat to a substrate toinduce a chemical reaction. The reaction occurs only in the heatedareas. The heat tends to diffuse to an area larger than the laser beamspot, making the resolution of the process poorer than the laser spotsize and causing concomitant thermal damage to nearby structures.Another mechanism used in laser micromachining is photochemical etching,in which the laser energy is absorbed by individual atoms or molecules(particles) of the substrate, exciting them into a state in which theycan chemically react with an etchant. Photochemical etching is limitedto materials that are photochemically active. Another mechanism used inlaser machining is laser ablation, in which energy supplied rapidly to asmall volume causes atoms to be explosively expelled from the substrate.Laser ablation using an ultrashort pulsed laser (UPL) is described, forexample, in U.S. Pat. No. Re. 37,585 to Mourou for “Method forControlling Configuration of Laser Induced Breakdown and Ablation.” UPLablation (UPLA) overcomes some of the limitations of the processesdescribed above.

Charged particle beams include ion beams and electron beams. Ions in afocused beam typically have sufficient momentum to micromachine byphysically ejecting material from a surface. Because electrons are muchlighter than ions, electron beams are typically limited to removingmaterial by inducing a chemical reaction between an etchant vapor andthe substrate. Ions beams typically are generated from a liquid metalion source or by a plasma ion source. The spot size of a chargedparticle beam depends on many factors, including the type of particlesand the current in the beam. A beam with low current can typically befocused to a smaller spot and therefore produce a smaller structure thana beam with high current, but a low current beam takes longer tomicromachine a structure than a high current beam.

Lasers are typically capable of supplying energy to a substrate at amuch higher rate than charged particle beams, and so lasers typicallyhave much higher material removal rates than charged particle beams.FIG. 1 is a schematic illustration of a prior art laser ablating asurface. When a high power pulsed laser 102 producing beam 103 isfocused onto a target material 104 and the laser fluence exceeds theablation threshold of the material, chemical bonds in the targetmaterial are broken and the material is fractured into energeticfragments, typically a mixture of neutral atoms, ions, clusters, andnano- and micro-particles creating a plasma plume 106 above the materialsurface. Since the material leaves the reaction zone as an energeticplasma, gas, and solid debris mixture, the ablation process resemblesexplosive evaporation of the material which propels material fragmentsup and away from the point where the laser is focused. As the plasmacools, much of the solid debris 108 is redeposited on the workpiecesurface, thus reducing the quality of the cut and decreasing the cuttingefficiency since the debris must be removed again before the beaminteracts with the workpiece surface.

Various techniques are known to minimize undesirable redeposition duringlaser ablation. For example, it is known to use an inert gas stream tocool the ablation site as described by Gua, Hongping et al. “Study ofGas-Stream Assisted Laser Ablation of Copper,” 218 THIN SOLID FILMS,274-276 (1992). U.S. Pat. No. 5,496,985 to Foltz et al. for “LaserAblation Nozzle,” describes the use of gas or fluid jets to removeejected material from the vicinity of the cut to prevent redeposition inthat area. Robinson, G. M. et al. “Femtosecond Laser Micromachining ofAluminum Surfaces Under Controlled Gas Atmospheres,” J. Mater. Eng. &Perf., Vol. 15(2), 155-160 (April 2006) describe the use of an inert gasas an inert gas shield. Such secondary techniques are often notcompletely effective, and add significant complexity to the laserablation system while decreasing cutting efficiency.

U.S. Pat. Pub. No. 2008/0241425 by Li et al. for “System and Method toReduce Redeposition of Ablated Material,” filed Oct. 2, 2008, describesa system where laser ablation is performed in a vacuum. According to Li,most laser ablation is performed in air (at normal atmospheric pressure)for low cost and convenience. Li teaches lowering the pressure in thesample chamber so that the ablated material will travel farther from themilling site before it loses a significant amount of kinetic energy andredeposits onto the surface. Unfortunately, even using the methoddescribed by Li, the material still redeposits onto the surface, some ofit just travels farther away than it would at higher chamber pressure.Further, the low pressure system described by Li makes it more likelythat the debris material will deposit onto various system components,such as lenses and pole pieces, as described below.

Like lasers, charged particle beam systems also have a problem withmaterial redeposition. In order to prevent significant redeposition andincrease the material removal rate, charged particle beam systems oftenmake use of gas-assisted etching (GAE). In GAE, an etching gas (alsoreferred to as a precursor gas) is directed at the material surface sothat a monolayer of gas particles (molecules or atoms depending on thetype of gas) is adsorbed onto the material surface. Irradiation of thematerial surface by a charged particle beam leads to the dissociation ofthe adsorbate, producing reactive fragments that react with the samplematerial to form volatile products that can be pumped away. Asignificant factor in the rate of material removal is the rate at whichgas particles are adsorbed on the surface. If a charged particle beam,such as a focused ion beam (FIB), dwells too long in one location, alladsorbates will be dissociated or desorbed and the beam will begin toremove material by sputtering, with the resulting problem of materialredeposition. While milling, the ion beam is typically scannedrepeatedly over a rectangle in a raster pattern. As the beam completes ascan, the beam is typically delayed for a significant amount of timebefore beginning the next scan to provide time for additional gasparticles to adsorb onto the surface before beginning a new raster. Thisincreases processing time. High concentrations (high gas pressures) ofthe precursor gas are not generally helpful because only a relativelysmall number of particles, forming a monolayer on the surface, adsorbonto the material surface at a time.

A similar gas-assisted etching process employing long pulse andcontinuous wave lasers is known as photochemical etching. PhotochemicalLaser Etching (PLE) involves directing a beam at the workpiece surfacewith an energy level below the ablation threshold of the material beingprocessed while the workpiece surface is exposed to a precursor gas.Instead of removing the material by the very rapid process of thermalablation described above, the laser only provides energy to the adsorbedgas particles causing the formation of a volatile compound thatchemically etches the surface. While PLE does prevent redepositionartifacts, the material removal rate using this process is a fraction ofthe rate using thermal ablation.

Laser-assisted chemical etching (LCE) is another well known techniquecombining nanosecond lasers and reactive gasses to etch certainsubstrates, such as silicon, in the presence of a high-pressure gas,such as chlorine. Such a process is described, for example, by DanielEhrlich et al., Laser Etching for Flip-Chip De-Bug and InverseStereolithography for MEMS, SOLID STATE TECH., June 2001, pp. 145-150(“Ehrlich”). In the process described by Ehrlich, however, the laser isnot used to ablate the sample surface; rather the laser is used to heatthe silicon until it becomes molten. The molten silicon then reacts withthe chlorine gas to etch the silicon. According to Ehrlich, as thechlorine reactant pressure is increased, etching (directly proportionalto the reaction flux) increases linearly for a time then saturates asthe nonlinear effect of gas diffusion limits long-range transfer ofreactant gas due to the formation of a depleted “boundary layer.” As aresult, etching rates (without redeposition) can only be increased to arelatively low rate when compared to the removal rates that are possibleusing thermal ablation.

What is needed is an improved method for laser processing that preventsmaterial redeposition during laser ablation but allows material to beremoved at a high rate.

SUMMARY OF THE INVENTION

An object of the invention is to provide an improved method for laserprocessing that prevents material redeposition during laser ablation butallows material to be removed at a high rate.

A preferred embodiment includes performing laser ablation in a chamberfilled with high pressure precursor (etchant) gas so that sampleparticles ejected during laser ablation will react with the precursorgas in the gas atmosphere of the sample chamber. When the ejectedparticles collide with precursor gas particles, the precursor isdissociated, forming a reactive component that binds the ablatedmaterial. In turn, the reaction between the reactive dissociationby-product and the ablated material forms a new, volatile compound thatcan be pumped away in a gaseous state rather than redepositing onto thesample.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter. It should be appreciated by those skilled in the art thatthe conception and specific embodiments disclosed may be readilyutilized as a basis for modifying or designing other structures forcarrying out the same purposes of the present invention. It should alsobe realized by those skilled in the art that such equivalentconstructions do not depart from the spirit and scope of the inventionas set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more thorough understanding of the present invention, andadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a beam system for use with the present invention;

FIG. 2 is a schematic illustration of a femtosecond laser ablating asurface according to the present invention;

FIG. 3 is a photomicrograph showing the results of laser ablation underthree different gas pressures according to a preferred embodiment of thepresent invention;

FIGS. 4A-D are photomicrographs showing the results of laser ablationfor boxes milled in a substrate at different etchant gas pressuresaccording to a preferred embodiment of the present invention;

FIG. 5 shows a beam system for use with the present invention; and

FIG. 6 shows a beam system using a smaller sample cell having a pressurelimiting aperture for use with the present invention.

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Although those of ordinary skill in the art will readily recognize manyalternative embodiments, especially in light of the illustrationsprovided herein, this detailed description is exemplary of the preferredembodiments of the present invention, the scope of which is limited onlyby the appended claims.

As described above, laser ablation is capable of rapidly removing amassive amount of material from a surface. Much of the material removed,however, is redeposited onto the surface, reducing the quality of thecut and decreasing cutting efficiency. As also described above,gas-assisted milling is used to reduce or eliminate redeposition incharged particle beam systems. Similar techniques have been employedwith laser beam systems. Due to the relatively massive amount ofmaterial removed during laser ablation, the familiar mechanism ofgas-assisted milling using a focused ion beam is of little or nobenefit. A monolayer of an etchant gas adsorbed onto the sample surfaceis removed so quickly by the laser that its effect on the milling rateis only marginal. Once the etchant gas is depleted, the laser will beginto ablate the surface as described above, resulting in significantmaterial redeposition.

Applicants have discovered that, instead of relying on the gas adsorbedonto the surface, it is possible to fill the sample chamber with a greatenough pressure of etchant gas that the ejected particles themselveswill react with the etchant gas in the gas atmosphere of the samplechamber. Rather than having the reaction limited to the surface layer ofthe sample and a monolayer of gas adsorbed on the surface, the presentinvention allows a relatively large volume of sample material to beejected from the sample and to react with the high concentration ofprecursor gas in the sample chamber. This allows for an enormousincrease in the material removal rates (due to laser ablation) whileeliminating the problems associated with material redeposition.

FIG. 2 is a schematic illustration of a femtosecond laser ablating asurface according to the present invention. In prior art gas-assistedetching in a charged particle beam system, the sample chamber ismaintained at a vacuum pressure of between approximately 1.33×10⁻⁵ Pa(1×10⁻⁷ Torr) and 6.66×10⁻² Pa (5×10⁻⁴ Torr). Once the etch-assistinggas is used, the chamber background pressure may rise, typically toabout 1.33×10⁻³ Pa (1×10⁻⁵ Torr). In contrast, the pressure of theprecursor gas inside the sample chamber 140 of FIG. 2 during laserablation is preferably much higher. As described in greater detailbelow, the actual pressure used will depend upon the gas species usedbut the pressure is preferably high enough that most of the ejectedparticles 108 will collide with gas particles 202 (molecules or atoms)in the atmosphere of the sample chamber 140. During laser ablation, thegas precursor is dissociated by some combination of the hot ablatedmaterial, ejected photoelectrons from the sample, high energy photons(X-rays) characteristic of ultra-fast pulsed laser ablation, or thefield generated by the focused laser beam. The reactive dissociationbyproduct then binds to the ablated material, forming a new, volatilecompound 204 which can be pumped away in gaseous form rather thanredeposited on the sample.

Thus, the reaction of the sample material is with gas particles in thegas phase, rather than gas particles adsorbed onto the sample surface.Even though some of the gas particles may be adsorbed onto the samplesurface, the amount of material removed during the laser ablation is sogreat that the adsorbed particles would only be able to volatilize asmall fraction of the material being removed. Instead of keeping theprecursor gas volume low, as in typical gas assisted etching, preferredembodiments of the present invention make use of a precursor gas at amuch higher pressure so that the ejected material will be more likely tocollide with the gas particles and produce the desired volatileby-product.

Persons of skill in the art will recognize that, even in prior artgas-assisted etching, some relatively small numbers of substrateparticles (molecules or atoms) may escape from the surface of thesubstrate and react with gas particles in the atmosphere of the vacuumchamber. As is well-known in the art, all solids exhibit a vaporpressure, although this vapor pressure is typically extremely low. Inprior art gas-assisted etching, the rate at which substrate particlesescape the surface and react with gas particles in the chamberatmosphere will be so low as to be inconsequential in relation to therate at which substrate particles react with adsorbed gas particles onthe substrate surface.

In contrast, according to the present invention, the reaction betweenthe substrate and the etching gas particles takes place primarily in theatmosphere of the chamber rather than on the surface. In other words,most of the sample material is first ejected from the sample surface vialaser ablation and then reacts with gas particles in the atmosphere ofthe sample chamber. Although some material on the sample surface willtypically also react with the etching gas, more of the sample materialremoved is volatilized in the gas atmosphere, than is volatilized on thesample surface. This is because the present invention is not limited tothe reaction between the substrate particles on the surface and themonolayer of etchant gas that comes into contact with those surfaceparticles. Instead, the laser ablation of the surface and the use ofhigher pressures of etchant gas allows for a much larger volume of thesample to react with the etchant gas at the same time. As used herein,the phrase “volatilized in the gas atmosphere” (or in the vacuum chamberatmosphere) will be used to refer to sample material that is ejectedfrom the sample surface and reacts to form a volatile compound insteadof redepositing onto the sample (or depositing onto another surface suchas the system components). According to preferred embodiments of thepresent invention, more than 80% of the sample material removed isvolatilized in the gas atmosphere rather than on the sample surface;more preferably, more than 90% of the sample material removed isvolatilized in the gas atmosphere.

Further, the formation of an adsorbed monolayer of etching gas on asample surface, as taught by the prior art, is temperature dependent.Sticking coefficient is the term used in surface physics to describe theratio of the number of adsorbate atoms (or molecules) that do adsorb, or“stick,” to a surface to the total number of atoms that impinge uponthat surface during the same period of time. Sometimes the symbol S_(c)is used to denote this coefficient, and its value is between 1.00 (allimpinging atoms stick) and 0.00 (none of the atoms stick). Thecoefficient is inversely proportional to the exponential of thetemperature. Above a certain temperature (which varies for each type ofgas) the sticking coefficient of the gas will reach 0.00 and none of thegas particles will be adsorbed onto the sample surface. Instead, all ofthe gas particles will be driven into the atmosphere. Prior artgas-assisted etching will not work under such conditions. The presentinvention, however, can still be used to etch such a sample withoutsignificant redeposition because the ejected material is volatilized inthe gas atmosphere rather than on the surface.

Persons of ordinary skill in the art will recognize that a higherprecursor gas pressure will make it more likely that the ejectedparticles will collide with gas particles and produce the desiredvolatile gas by-product. Persons of ordinary skill will be able toeasily determine the lowest gas pressure which produces a suitabledecrease in redeposition. Preferably, the sample to be laser ablated isplaced in an atmosphere of a precursor gas having a pressure typicallybetween 13 Pa (0.1 Torr) and 6666 Pa (50 Torr), and more typicallybetween 133 Pa (1 Torr) and 1333 Pa (10 Torr). In some preferredembodiments, a high enough gas pressure can be used so thatsubstantially all of the ejected material is volatilized rather thanredeposited. In this context, the phrase “substantially all” is used toindicate that the great majority of the ejected material is volatilizedso that none or only an insignificant about of the sample material isredeposited. Preferably, more than 80% of the ejected material isvolatilized; more preferably, more than 90% is volatilized.

Likewise, the phrase “a significant reduction in redeposition” orsimilar phrasing will be used to compare redeposition using laserablation without a precursor gas to the much smaller amount ofredeposition, especially in the milled area, that results from laserablation in the presence of a precursor gas at a suitable pressure. Inpreferred embodiments, redeposition in the sample area will be reducedby more than 50%; more preferably by more than 80%; and even morepreferably by more than 90%.

While in general higher pressures are more desirable, there are factorsthat may limit the desirable precursor gas levels. In other preferredembodiments, a desired gas pressure could result in a smaller percentageof ejected material being volatilized in the gas atmosphere. Forexample, in some applications, a smaller reduction in redeposition couldbe acceptable—such as 70% or even 50% reduction in redeposition ascompared to ablation without using the precursor gas. Obviously, highergas pressures will physically use more gas, raising material and storagecosts. For such applications, the use of a lower concentration ofetching gas, which would serve to lower material and storage costs,might be desirable.

Further, some commonly used precursor gasses have vapor pressures thatlimit the concentrations that can be used. XeF₂, for example, has avapor pressure of approximately 533.2 Pa (4 Torr) at room temperature.Consequently, where XeF₂ is used as a precursor gas, it would bedesirable to keep the pressure below 533.2 Pa to prevent the gas fromcondensing. Other precursor gases can be used at pressures up toatmospheric pressure or beyond, although as described below, a higherpressure will take longer to pump down for charged particle beam imagingor processing. Various precursor gases could be used, depending on thematerial being processed, for example, Cl₂, I₂, SiF₄, CF₄, NF₃ for Si,N₂O, NH₃+O₂, or NO₂ for Cu.

Also, for precursor gases with a lower ionization energy, higher gaspressures could cause gas ionization by a laser beam of sufficientlyhigh energy, which could interfere with the laser energy reaching theworkpiece surface thus slowing down the ablation process. Where theprecursor gas has an ionization energy above the energy of the unfocusedlaser beam, however, even at higher pressures the precursor gas will notreact with the beam at all. The focused beam will dissociate gases withhigh ionization energies due to nonlinear effects resulting from theextremely high field generated by ultra-short pulses.

FIG. 3 illustrates the effects of using a precursor gas with laserablation in accordance with a preferred embodiment of the presentinvention. The three rectangular trenches (302, 304, and 306) shown inFIG. 3 were machined in a sample of SiO₂ using a 150 femtosecondTi:Sapphire laser having a central wavelength of 775 nm. Trench 302 wasmilled in a typical high vacuum of approximately 1.33×10⁻¹ Pa (1×10⁻³Torr). Significant redeposition artifacts are clearly visible on thesides and floor of box 302. Trench 304, however, was milled after theintroduction of XeF₂ gas at a pressure of 266 Pa (2 Torr). Redepositionartifacts are greatly reduced in trench 304. The sides of trench 304appear substantially vertical, unlike the heavily sloped sidewalls oftrench 302. Finally, trench 306 was milled after the XeF₂ gas had beenremoved and the chamber pumped back down to a pressure of approximately1.33×10⁻¹ Pa (1×10⁻³ Torr). Not surprisingly, trench 306 shows the sameredeposition artifacts seen in trench 302.

Note that the precursor gas pressure used while milling 304 would beclose to optimal for many uses. While there does appear to be someredeposition on the substrate surface around trench 304, there is a muchsmaller amount of redeposited material inside the sample area beingmilled. A higher precursor gas pressure might result in less totalredeposition, but because the redeposition is already outside the samplearea, for many uses the additional material costs for a greater amountof precursor gas would not be justified.

FIGS. 4A-4D illustrate the results of laser ablation under threedifferent precursor gas pressures in accordance with the presentinvention. For all images, the average power of the delivered laser beamwas 1.8 mW at 1 kHz, or 1.8 μJ per pulse. Features made in the substrateby the focused laser beam were measured to be 4.25±0.25 μm for n>1000pulses. The laser beam was directed onto the substrate at normalincidence and the stage was scanned in the X (horizontal) direction at aconstant velocity of 50 μm/sec. Between each horizontally scanned line,the stage was stepped 1 μm in the Y (vertical) direction and the scandirection reversed. A total of 100 lines were machined for a final pithaving nominal dimensions of 100 μm×100 μm. In all images, the scanstarted in the upper left-hand corner, proceeded horizontally, and endedin the lower left hand corner.

FIG. 4A shows a rectangular hole milled in the substrate using laserablation at a high vacuum (approximately 5.9×10-3 Pa) without using aprecursor gas. As seen in the image, a significant amount of the ablatedmaterial redeposited into the hole 402. FIG. 4A illustrates how materialejected during ablation redeposits in a radial distribution with littlevariation through 2π radians. Material ejected in the direction of thescan is removed as the scan progresses, and accumulates preferentiallyat the trailing edge of the scan pattern. Thus, the redeposition seen inFIG. 4A is most pronounced at the top edge and almost completely absentfrom the bottom edge of the ablation pit.

FIGS. 4B-4D show rectangular holes milled in the substrate using laserablation using XeF₂ as a precursor gas at pressures 67, 267, and 533 Pa,respectively. As clearly illustrated by these images, the presence ofXeF₂ results in a significant reduction of redeposited material. Thebottom and sidewalls of the milled holes of FIGS. 4B-4D are notsignificantly distorted by recast material, in contrast to the holemachined in high vacuum as shown in FIG. 4A.

The distance D at the upper corner of FIGS. 4B-4D is used as a measureof redeposition. The apparent thickness (D1, D2, and D3 respectively) ofthe trailing edge of the scan pattern (i.e., top edge of each hole) canbe used as an approximate measure of the extent of redeposition. In theimages, D is seen to decrease with increasing XeF₂ pressure. The valuesof D1 (FIG. 4B), D2 (FIG. 4C), and D3 (FIG. 4D) are 10.34, 6.85, and5.83 μm, respectively. This decrease is attributed to a correspondingincrease in the amount of ejected material converted to volatile speciessuch as SiF_(x) and O₂ during ablation, and the subsequent removal ofthese gas molecules by the pumping system.

As discussed above, at lower pressures of XeF₂ the methods describedherein will still result in a significant reduction in redeposition (dueto volatilization of the ejected sample material in the gas atmosphere)but the reduction will be lower. For example, if the pressure of XeF₂ isreduced from 266 Pa (2 Torr) to 66 Pa (0.5 Torr) or even 33 Pa (0.25Torr), the amount of sample volatilized in the gas atmosphere might bereduced by 10% or 20%. In other words, the reduction in redepositionscales with the precursor gas pressure.

FIG. 5 shows a system 500 for use with a preferred embodiment of thepresent invention that combines a laser with a charged particle beam formonitoring the ablation process or for further material processing. Alaser 502 directs a beam 503 to a sample 504. Sample 504 may include,for example, a single crystal, Z-cut SiO₂ substrate. Laser 502 ispreferably capable of being operated at a fluence greater than theablation threshold of the material being machined. Embodiments of theinvention could use any type of laser, now existing or to be developed,that supplies sufficient fluence. A preferred laser provides a short,that is, nanosecond to femtosecond, pulsed laser beam. Suitable lasersinclude, for example, a Ti:Sapphire oscillator or amplifier, afiber-based laser, or a ytterbium or chromium doped thin disk laser.

For example, a preferred laser system may include a Ti:Sapphire chirpedpulse amplification system capable of delivering 150 fs, 1 mJ pulseswith a center wavelength of 775 nm, and at a repetition rate of 1 kHzfor a total average power of 1 W. Active power modulation is preferablyachieved by a rotating half waveplate and a fixed linear polarizer. Aneutral density filter preferably provides further attenuation and aniris is used to adjust the beam diameter. The beam is preferably focusedonto the substrate by an infinity corrected 20× microscope objectivewith an NA of 0.40 such as one available, for example, from MitutoyoAmerica Corporation. Preferably, average power readings of the beam aremade using a silicon detector located between the microscope objectivelens and the specimen chamber. In one preferred embodiment, the beamenters the chamber through an O-ring sealed, 5 mm thick, BK7 glassoptical window. A chamber for use in one preferred embodiment has a basepressure of approximately 5×10⁻³ Pa and employs a heater (although notnecessary), thermocouple, and leak valves to control substratetemperature and gas pressure. Pressure is preferably measured by gasspecies independent capacitance manometers.

Sample 504 is typically positioned on a precision stage 509, whichpreferably can translate the sample in the X-Y plane, and morepreferably can also translate the work piece in the Z-axis, as well asbeing able to tilt and rotate the sample for maximum flexibility infabricating three dimensional structures. The stage may also be a heatedstage. System 500 optionally includes one or more charged particle beamcolumns 530, such as an electron beam column, an ion beam column, orboth, which can be used for imaging the sample to monitor the laserablation process for other processing or imaging tasks. Charged particlebeam 530 typically includes a source 532 of charged particles; afocusing column 534 for forming a beam of charged particles from thesource of charged particles and for focusing and scanning the beam ofcharged particles onto the substrate surface; a secondary particledetector 536, typically a scintillator-photomultiplier detector, forforming an image of the sample 504; and a gas injection 538 system forsupplying a precursor gas that reacts in the presence of the chargedparticle beam. System 500 may also include an atomic force microscope(ATM) (not shown).

Sample chamber 501 preferably includes one or more gas outlets forevacuating the sample chamber using a turbomolecular and mechanicalpumping system under the control of a vacuum controller. Sample chamber501 also preferably includes one or more gas inlets through which gascan be introduced to the chamber at a desired pressure. In a preferredembodiment, before the laser ablation is performed, sample chamber 501is first evacuated to a pressure of 1.33×10⁻¹ Pa (1×10⁻³ Torr) or less.A precursor gas is then introduced until the pressure in the chamberreaches some predetermined value. For example, XeF₂ could be introduceduntil the pressure in the sample chamber reaches at least 266 Pa (2Torr); more preferably, the XeF₂ pressure will be increased until thepressure in the sample chamber reaches 66.6 to 133.3 Pa (0.5-1.0 Torr).

Laser ablation is then performed by directing the laser toward thesubstrate at an area to be micromachined. The ablation threshold is anintrinsic property of the substrate material, and skilled persons canreadily determine empirically or from the literature the ablationthreshold for various materials. A silicon substrate, for example, has asingle pulse ablation threshold of about 170 mJ/cm2, and so the laserfluence should preferably be just above this value for micromachiningsilicon in accordance with the invention. A preferred laser beam hasenergy in the range of 10 nJ to 1 mJ, and a fluence in the range of 0.1J/cm2 to 100 J/cm2. In one preferred embodiment for milling a siliconsubstrate, the laser beam has a fluence of 190 mJ/cm2, a pulse durationof 150 femtoseconds, and a spot size of 2 μm. In another embodiment, alaser beam has a pulse energy of 50 nJ and a fluence of 0.4 J/cm2.

As described above, the actual pressure of the precursor gas will dependupon the gas species used but the pressure will preferably be highenough that substantially all of the ejected particles will collide withgas particles in the atmosphere of the gas chamber thus generating achemically active species that volatilizes the ejected material inflight as it leaves the sample.

In the preferred embodiment of FIG. 5, when charged particle beam column530 or secondary particle detector 536 are used, the substrate must bemaintained in a vacuum. As described below, the precursor gas would bepumped out and the sample chamber restored to vacuum before the chargedparticle beam is employed. In some embodiments, detector 536 can be usedto detect a secondary electron signal to form an image, which can beused to monitor the laser ablation process. A computer 520 controls thesystem 500 and a display 522 displays an image of the sample for theuser. In some embodiments, an additional detector 510 detects emissions,such as x-rays or other photons from the sample 504 to determine when amachining process is complete. Endpointing processes for use with lasermachining are described in U.S. Pat. App. No. 61/079,304 for Jul. 9,2008, for “Method and Apparatus for Laser Machining,” which is herebyincorporated by reference.

In addition to an improvement in cut quality via the reduction ofredeposited material, the present invention has the added benefit ofreducing contamination to system components (such as lenses and polepieces). There are two mechanisms responsible for this effect. First, bycombining with the reactive dissociation byproduct, much of the ablatedmaterial is volatilized and pumped away before being deposited on systemcomponents. Secondly, because of the relatively high gas pressuresinvolved, the mean free path of the ablated material that does not reactwith the dissociation byproducts is reduced. As a result, the particlesin question travel shorter distances before coming to rest, never cominginto contact with system components. Evidence of this effect can be seenin the increased debris field around box 304 in FIG. 3.

One disadvantage of the apparatus of FIG. 5 is that the high gaspressure used during laser ablation is not suitable for use with acharged particle beam column, such as a typical dual beam system or SEM.In order to use the charged particle beam, for example, to monitor thematerial removal process, the sample chamber would have to be pumped outto a suitable vacuum. The higher the pressures used for the laser, thelonger it will take to pump out the pressure to make use of the SEM orion beams. The time required to pump out and re-pressurize the chamberwould be significant.

Instead of pressurizing the entire sample chamber with the precursorgas, in some preferred embodiments, the sample can be placed inside asmaller sample cell within the main sample chamber as described in U.S.patent application Ser. No. 12/525,908 for “High Pressure ChargedParticle Beam System,” filed Aug. 5, 2009, which is assigned to theassignee of the present invention and incorporated herein by reference.FIG. 6 shows a preferred embodiment of the present invention making useof such a sample cell.

Preferred embodiments of the invention use a cell in which a sample ispositioned for charged particle beam processing. A pressure limitingaperture maintains a lower pressure outside of the cell. A cell can bepositioned inside a conventional high vacuum SEM chamber to provide thecapability of surrounding the sample with a higher pressure precursorgas atmosphere while maintaining a suitable vacuum inside the focusingcolumn and main sample chamber. Gas particles scatter the primaryelectron beam, and so the pressure limiting aperture is positioned tominimize the distance that the electron beam travels in the highpressure region to reduce interference with the primary beam, whileproviding a sufficient travel distance for adequate gas amplification ofthe secondary electron signal, as described below.

The volume of the cell is typically significantly smaller than thevolume of a typical prior art sample chamber, thereby reducing thequantity of precursor gas required to achieve a desired pressure forlaser processing. Because the quantity of gas in the cell is relativelysmall, gas can be introduced, evacuated, and distributed within a cellmore quickly than with a conventional sample chamber. Containing the gaswithin a cell protects the sample chamber and the electron focusingcolumn from any adverse affects, such as corrosion, from processinggases, while the cell can be constructed from materials that will not beadversely affected by the gases. A cell can be disposable, which can beadvantageous when an extremely reactive gas is used. Further, the use ofa smaller sample cell within the main sample chamber has the addedadvantages of reducing overall gas consumption and protecting varioussystem components from exposure to corrosive precursor gases andchemicals.

In the preferred embodiment of FIG. 6, system 600 includes laser 502 andcharged particle beam 540. Sample cell 616 fits inside a larger samplechamber 401 and has a pressure limiting aperture (PLA) 614. The interiorvolume and surface area of the sample cell 316 are preferably smallrelative to the sample chamber 501.

System 600 also includes a sample cell gas inlet 620, an inlet leakvalve 622, a sample cell gas outlet 624, and an outlet leak valve 626,which can be vented to the sample chamber 501 as shown, or to a roughingpump (not shown). This allows for rapid filing and evacuation of thecell. The pressure in the cell is a result of the flow rate into thecell through the gas inlet, the flow rate out of the cell through thegas outlet, and the leakage of gas through the PLA. Upper PLA 614 limitsgas flow from the interior portion of sample cell 616 into the focusingcolumn. PLA 614 preferably has a diameter of 800 micrometers or less.

A secondary electron detector 640 extends into sample chamber 501 andterminates at a position above PLA 614. The detector 640 is in the formof a needle and is suitable for use at relatively high pressures. Inprior art High Pressure SEMs, secondary electrons are typically detectedusing a process known as “gas amplification,” in which the secondarycharged particles are accelerated by an electric field and collide withgas particles in an imaging gas to create additional charged particles,which in turn collide with other gas particles to produce stilladditional charged particles. This cascade continues until a greatlyincreased number of charged particles are detected as an electricalcurrent at a detector electrode. In some embodiments, each secondaryelectron from the sample surface generates, for example, more than 20,more than 100, or more than 1,000 additional electrons, depending uponthe gas pressure and the electrode configuration.

A housing stage 509, such as a conventional SEM sample stage, inside thesample chamber 501 is used for tilting or aligning cell 616 so that PLA614 can be aligned with the axis of charged particle beam 530 or oflaser beam 503. A cell sample stage 608 is also included inside the cell616, for aligning a region of interest on sample 504 under the axis ofelectron beam 530 or of laser beam 503 for processing. While theembodiment of FIG. 6 allows the laser to enter the cell 616 through thePLA 614, in other embodiments, the laser beam could enter the cell 616by other methods. For example, the laser could enter the cell through awindow that is transparent to the laser wavelength. The window could bepositioned, for example, on the top or side of the cell. The laser beamcould also be routed into the cell using a photonic crystal fiber havingnegative group-velocity dispersion.

The use of a sample cell as shown in FIG. 6 would allow the use of acharged particle beam, such as an electron beam, to monitor the processof laser ablation in real time without having to pump down the entiresample chamber. In the preferred embodiment shown in FIG. 6, the SEMcould still be used with the gas pressure in the sample cell as high as2666 Pa (20 Torr). For gas pressures in the sample cell above 2666 Pa itwould be preferable to pump down the gas pressure before activating theelectron beam.

In another embodiment of the present invention, the beam can be focusedabove the sample in a precursor gas environment. The intense fieldproduced by the focused laser pulse will dissociate gas phase precursorparticles forming reactive byproducts. In a fashion similar to thatdescribed above, these byproducts combine with atoms in the substrate toform volatile compounds resulting in a photon induced chemical etch.

Although the description of the present invention above is mainlydirected at a method of removing material from a sample by laserablation while reducing redeposition, it should be recognized that anapparatus performing the operation of this method would further bewithin the scope of the present invention. Further, it should berecognized that embodiments of the present invention can be implementedvia computer hardware or software, or a combination of both. The methodscan be implemented in computer programs using standard programmingtechniques—including a computer-readable storage medium configured witha computer program, where the storage medium so configured causes acomputer to operate in a specific and predefined manner—according to themethods and figures described in this Specification. Each program may beimplemented in a high level procedural or object oriented programminglanguage to communicate with a computer system. However, the programscan be implemented in assembly or machine language, if desired. In anycase, the language can be a compiled or interpreted language. Moreover,the program can run on dedicated integrated circuits programmed for thatpurpose.

Further, methodologies may be implemented in any type of computingplatform, including but not limited to, personal computers,mini-computers, main-frames, workstations, networked or distributedcomputing environments, computer platforms separate, integral to, or incommunication with charged particle tools or other imaging devices, andthe like. Aspects of the present invention may be implemented in machinereadable code stored on a storage medium or device, whether removable orintegral to the computing platform, such as a hard disc, optical readand/or write storage mediums, RAM, ROM, and the like, so that it isreadable by a programmable computer, for configuring and operating thecomputer when the storage media or device is read by the computer toperform the procedures described herein. Moreover, machine readablecode, or portions thereof, may be transmitted over a wired or wirelessnetwork. The invention described herein includes these and other varioustypes of computer-readable storage media when such media containinstructions or programs for implementing the steps described above inconjunction with a microprocessor or other data processor. The inventionalso includes the computer itself when programmed according to themethods and techniques described herein.

Computer programs can be applied to input data to perform the functionsdescribed herein and thereby transform the input data to generate outputdata. The output information is applied to one or more output devicessuch as a display monitor. In preferred embodiments of the presentinvention, the transformed data represents physical and tangibleobjects, including producing a particular visual depiction of thephysical and tangible objects on a display.

To the extent that any term is not specially defined in thisspecification, the intent is that the term is to be given its plain andordinary meaning. The accompanying drawings are intended to aid inunderstanding the present invention and, unless otherwise indicated, arenot drawn to scale.

A preferred method or apparatus of the present invention has many novelaspects, and because the invention can be embodied in different methodsor apparatuses for different purposes, not every aspect need be presentin every embodiment. Moreover, many of the aspects of the describedembodiments may be separately patentable. The terms “workpiece,”“sample,” and “specimen” are used interchangeably in this application.As used herein, the terms “ablation,” “laser ablation,” and “thermalablation” is used to refer to material removal by means of laserradiation. “Photochemical laser etching” is used to refer to materialremoval by way of a laser activated chemical reaction. As used herein,the term “gas particles” will be used to refer to both molecules andatoms.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made to the embodiments described herein withoutdeparting from the spirit and scope of the invention as defined by theappended claims. Moreover, the scope of the present application is notintended to be limited to the particular embodiments of the process,machine, manufacture, composition of matter, means, methods and stepsdescribed in the specification. As one of ordinary skill in the art willreadily appreciate from the disclosure of the present invention,processes, machines, manufacture, compositions of matter, means,methods, or steps, presently existing or later to be developed thatperform substantially the same function or achieve substantially thesame result as the corresponding embodiments described herein may beutilized according to the present invention. Accordingly, the appendedclaims are intended to include within their scope such processes,machines, manufacture, compositions of matter, means, methods, or steps.

We claim as follows:
 1. A method of removing material from a sample bylaser ablation while reducing redeposition, the method comprising:providing an apparatus for laser micromachining having a vacuum chamberfor holding a sample, a source of a precursor gas, and a laser systemfor operating on the sample in the vacuum chamber, the laser systemgenerating a pulsed laser beam having an energy great enough to ablatethe sample; loading a sample into the vacuum chamber; filling the vacuumchamber with a desired concentration of precursor gas to form anatmosphere of precursor gas particles in the vacuum chamber around thesample, the precursor gas being a gas that will react with the samplematerial, when sufficient energy is provided to initiate said reaction,to form a volatile compound that will not redeposit onto the samplesurface; and directing the laser at the sample to ablate the surface,the laser operated at a fluence greater than the ablation threshold ofthe sample material so that sample particles are ejected into theprecursor gas atmosphere in the vacuum chamber; the laser providingsufficient energy to the ejected sample particles to initiate thereaction with the precursor gas particles: wherein the desiredconcentration of precursor gas is high enough that that the volume ofejected particles that collide with and react with gas particles and arethereby volatilized is high enough to significantly reduce redepositiononto the sample and wherein at least 80% of the sample material ejectedfrom the sample surface via laser ablation is volatilized in the vacuumchamber atmosphere so that it does not redeposit.
 2. (canceled) 3.(canceled)
 4. (canceled)
 5. The method of claim 1 in which at least 90%of the sample material ejected from the sample surface via laserablation is volatilized in the vacuum chamber atmosphere so that it doesnot redeposit.
 6. The method of claim 1 where the majority of materialremoved from the surface is ejected from the sample surface via laserablation and volatilized in the vacuum chamber atmosphere so that itdoes not redeposit.
 7. (canceled)
 8. (canceled)
 9. (canceled) 10.(canceled)
 11. The method of claim 1 in which the desired concentrationof precursor gas is between 133 Pa and 1333 Pa.
 12. (canceled) 13.(canceled)
 14. (canceled)
 15. (canceled)
 16. (canceled)
 17. (canceled)18. (canceled)
 19. The method of claim 1 in which the sample comprisesSiO₂.
 20. (canceled)
 21. (canceled)
 22. (canceled)
 23. (canceled) 24.(canceled)
 25. (canceled)
 26. (canceled)
 27. (canceled)
 28. (canceled)29. (canceled)
 30. An apparatus for laser micromachining adapted tominimize redeposition of material removed from a sample by laserablation, the apparatus comprising: a vacuum chamber for holding asample; a source of a precursor gas for filling the vacuum chamber withsaid precursor gas to a desired pressure; a laser system for operatingon the sample in the vacuum chamber, the laser system generating apulsed laser beam having an energy great enough to ablate the sample sothat material removed from the surface is ejected into the gasatmosphere where at least a portion of the ejected material reacts withgas particles to form a volatile compound that will not redeposit ontothe sample surface; and the desired gas pressure providing an adequateconcentration of gas particles in the vacuum chamber so thatsubstantially all of the ejected material is volatilized in theatmosphere of the vacuum chamber before redeposition.
 31. The method ofclaim 30 further comprising a charged particle beam column.
 32. Themethod of claim 30 in which the laser system includes a lens.
 33. Themethod of claim 1 in which the laser system includes a lens.